The production of alkylene oxides via catalytic epoxidation of olefins in the presence of oxygen using silver based catalysts is known. Conventional silver-based catalysts used in such processes typically provide a relatively lower efficiency or “selectivity” (i.e., a lower percentage of the reacted alkylene is converted to the desired alkylene oxide). In certain exemplary processes, when using conventional catalysts in the epoxidation of ethylene, the theoretically maximal efficiency towards ethylene oxide, expressed as a fraction of the ethylene converted, does not reach values above the 6/7 or 85.7 percent limit. Therefore, this limit had long been considered to be the theoretically maximal efficiency of this reaction, based on the stoichiometry of the following reaction equation:7C2H4+6O2→6C2H4O+2CO2+2H2O
cf. Kirk-Othmer's Encyclopedia of Chemical Technology, 4th ed., Vol. No. 9, 1994, p. 926.
Certain “high efficiency” or “high selectivity” modern silver-based catalysts are highly selective towards alkylene oxide production. For example, when using certain modern catalysts in the epoxidation of ethylene, the theoretically maximal efficiency towards ethylene oxide can reach values above the 6/7 or 85.7 percent limit referred to, for example 88 percent or 89 percent, or above. As used herein, the terms “high efficiency catalyst” and “high selectivity catalyst” refer to a catalyst that is capable of producing an alkylene oxide from the corresponding alkylene and oxygen at an efficiency greater than 85.7 percent. The observed actual efficiency of a high efficiency catalyst may fall below 85.7 percent under certain conditions based on process variables, catalyst age, etc. However, if the catalyst is capable of achieving at least an 85.7 percent efficiency, it is considered to be a high efficiency catalyst. Such highly efficient catalysts, which may comprise as their active components silver, rhenium, at least one further metal, and optionally, a rhenium co-promoter, are disclosed in EP0352850B1 and in several subsequent patent publications. “Promoters,” sometimes referred to as “inhibitors” or “moderators,” refer to materials that enhance the performance of the catalysts by either increasing the rate towards the desired formation of alkylene oxide and/or suppressing the undesirable oxidation of olefin or alkylene oxide to carbon dioxide and water, relative to the desired formation of alkylene oxide. As used herein, the term “co-promoter” refers to a material that—when combined with a promoter—increases the promoting effect of the promoter. In addition, promoters may also be referred to as “dopants.” In the case of those promoters that provide high efficiencies, the terms “high efficiency dopants” or “high selectivity dopants” may be used.
“Promoters” can be materials that are introduced to catalysts during the preparation of the catalysts (solid phase promoters). In addition, “promoters” can also be gaseous materials that are introduced to the epoxidation reactor feed (gas phase promoters). In one example, an organic halide gas phase promoter may be added continuously to the epoxidation reactor feed to increase the catalyst efficiency. For silver-based ethylene epoxidation catalysts, both solid and gas phase promoters are typically required in any commercial processes.
Conventional catalysts have relatively flat efficiency curves with respect to the gas phase promoter concentration in the feed, i.e., the efficiency is almost invariant (i.e., the change in efficiency with respect to a change in gas phase promoter concentration in the feed is less than 0.1%/ppm) over a wide range of promoter concentrations, and this invariance is substantially unaltered as reaction temperature is changed (i.e., the change in efficiency with respect to a change in reaction temperature is less than 0.1%/° C.) during prolonged operation of the catalyst. However, conventional catalysts have nearly linear activity decline curves with respect to the gas phase promoter concentration in the feed, i.e., with increasing gas phase promoter concentration in the feed, temperature has to be increased or the alkylene oxide production rate will be reduced.
By contrast, high efficiency catalysts tend to exhibit relatively steep efficiency curves as a function of gas phase promoter concentration as the concentration moves away from the value that provides the highest efficiency (i.e., the change in efficiency with respect to a change in gas phase promoter concentration is at least 0.2%/ppm when operating away from the efficiency maximizing concentration). Thus, small changes in the promoter concentration can result in significant efficiency changes, and the efficiency exhibits a pronounced maximum, i.e., an optimum, at certain concentrations (or feed rates) of the gas phase promoter for a given reaction temperature and catalyst age as well as other conditions such as feed gas composition. Moreover, the efficiency curves and the optimum gas phase promoter concentration tend to be strong functions of reaction temperature and are thus significantly affected if reaction temperature is varied, for example, to compensate for decreases in catalyst activity, (i.e., the change in efficiency with respect to a change in reaction temperature can be at least 0.1%/° C. when operating away from the efficiency maximizing promoter concentrations for the selected temperatures). In addition, high efficiency catalysts have exhibited significant activity increases with increases in the gas phase promoter concentration in the feed, i.e., with increasing gas phase promoter concentration in the feed, temperature has to be decreased or the production rate will increase.
Many commercial alkylene oxide processes are operated to achieve a targeted value of an alkylene oxide production parameter, such as the concentration of the alkylene oxide in the reaction product, alkylene oxide production rate, alkylene oxide production rate/catalyst volume (also known as the alkylene oxide “work rate”), alkylene oxide yield, alkylene conversion, and oxygen conversion. In order to maximize conversion of the alkylene, many known processes maintain the maximum reactor feed gas oxygen concentration that is allowable based on feed gas flammability considerations. Neither the alkylene nor oxygen are stoichiometrically limiting (i.e., neither of them are completely converted), and some amount of each is contained in the reaction product. When a reduction in the alkylene oxide production parameter is desired, reaction temperature is frequently reduced. The reduction in reaction temperature reduces the overall rate of consumption of alkylene and oxygen. However, it can also cause a significant shift in the efficiency of the process, resulting in the excessive generation of the unwanted byproducts carbon dioxide and water. Moreover, in certain cases, temperature cannot be used to control a desired alkylene oxide production parameter value. Alkylene oxide formation reactions are typically exothermic and require a coolant system to maintain a desired reaction temperature. The minimum achievable reaction temperature may be limited by the design and operability of the coolant system and may be higher than the temperature needed for the target production, resulting in overproduction of the alkylene oxide beyond that which is economically desirable or resulting in limitations in downstream recovery sections, such as a distillation section. While it may be possible to reduce gas phase promoter concentration to avoid such overproduction, underchloriding the catalyst can irreversibly impair catalyst efficiency. In addition, gas phase promoter concentrations are sometimes difficult to control to a degree necessary to maintain a desired alkylene oxide production parameter value due to their relatively low flow rates and the lack of sensitivity of available flow control systems. The problem tends to be more acute in the period following start-up when the activity of high efficiency catalysts tends to be high and/or the alkylene production target is low.